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| United States Patent |
6,111,151
|
|
Ewing
, et al.
|
August 29, 2000
|
Removal of water from process streams
Abstract
Process for removing water from compositions containing water, organics and
hydrogen fluoride (e.g. product stream from vapour phase hydrofluorination
processes, e.g. manufacture of HFA 134a, HFA 125 or HFA 32) by phase
separation and purge of the aqueous layer.
| Inventors:
|
Ewing; Paul Nicholas (Warrington, GB);
Bujac; Paul David Bernard (Taporley, GB);
Bonniface; David William (Warrington, GB)
|
| Assignee:
|
Imperial Chemical Industries PLC (London, GB)
|
| Appl. No.:
|
242053 |
| Filed:
|
February 8, 1999 |
| PCT Filed:
|
August 5, 1997
|
| PCT NO:
|
PCT/GB97/02104
|
| 371 Date:
|
February 8, 1999
|
| 102(e) Date:
|
February 8, 1999
|
| PCT PUB.NO.:
|
WO98/06685 |
| PCT PUB. Date:
|
February 19, 1998 |
Foreign Application Priority Data
| Current U.S. Class: |
570/177; 570/142; 570/165 |
| Intern'l Class: |
C07C 017/38; C07C 019/08; C07C 017/08 |
| Field of Search: |
570/177,165,142
|
References Cited
U.S. Patent Documents
| 5334784 | Aug., 1994 | Blake et al. | 570/177.
|
| Foreign Patent Documents |
| 2056977 | Mar., 1981 | GB.
| |
| 93/11853 | Jun., 1993 | WO.
| |
Primary Examiner: Siegel; Alan
Attorney, Agent or Firm: Cook, Alex, McFarron, Manzo, Cummings & Mehler, Ltd.
Claims
What is claimed is:
1. A process for the removal of water from a process stream which includes
hydrogen fluoride, water, organic hydrofluorocarbon products and
by-products and unreacted organic starting materials which comprises (i)
separating a lighter tops stream comprising hydrogen fluoride and lighter
boiling organic components from a heavy bottoms stream comprising hydrogen
fluoride, water and heavier organic components characterized in that (ii)
the heavy stream is fed to a phase separator under conditions of
temperature and pressure such that the heavy stream is in the liquid phase
and separating an organic fraction from a hydrogen fluoride fraction
containing water and (iii) disposing of at least a part of the hydrogen
fluoride fraction outside of the process.
2. A process according to claim 1 which comprises separating the hydrogen
fluoride fraction containing water into a recycle stream and a further
hydrogen fluoride stream containing water.
3. A process according to claim 2 in which further hydrogen fluoride is
added to the recycle stream.
4. A process according to claim 2 in which the relative proportions of the
fraction which is separated as the further hydrogen fluoride stream and as
the recycle stream are such that the concentration of water in the process
stream prior to separation into the lighter tops and heavy bottoms stream
is maintained at a level of less than 0.5% by weight.
5. A process according claim 1 in which the process stream is obtained by
the catalytic vapour phase hydrofluorination of a halogenated alkane or
halogenated alkene.
6. A process for the production of a hydrofluorocarbon which comprises
contacting a precursor compound with hydrogen fluoride in the vapour phase
in the presence of a hydrofluorination catalyst to produce a product
stream comprising the hydro-fluorocarbon, organic by-products, hydrogen
fluoride and water and treating at least part of the product stream, in a
process according to claim 1.
7. A process according to claim 6 in which substantially all of the product
stream is treated.
8. A process according to claim 6 in which the hydrofluorocarbon comprises
1,1,1,2 tetrafluoroethane and the precursor comprises
1,1,1-trifluoro-2-chloroethane and/or trichloroethylene.
9. A process according to claim 6 in which the hydrofluorocarbon comprises
pentafluoroethane and the precursor comprises perchloroethylene.
10. A process according to claim 6 in which the hydrofluorocarbon comprises
difluoromethane and the precursor comprises an .alpha.-fluoroether.
11. A process according to claim 10 in which the ether comprises
bis-fluoromethylether.
12. A process according to claim 10 in which the precursor comprises
methylene chloride.
Description
This invention relates to a process for the removal of water from process
streams and in particular to a process for the removal of water from
process steams which are generated during vapour phase catalytic
hydrofluorination reactions which employ hydrogen fluoride as the
hydrofluorinating reactant.
Recently much attention has been directed at the conception and development
of process routes for the production of hydrofluoroalkanes (HF As) which
have been proposed as replacements and indeed are now produced and sold as
replacements for chlorofluorocarbons.
Amongst the many processes which have been proposed for the production of
hydrofluoroalkanes, for example pentafluoroethane (HF A 125),
1,1,1,2-tetrafluoroethane (HFA 134a), difluoromethane (HFA 32) and
1,1,1-trifluoroethane (HFA 143a), vapour phase catalytic hydrofluorination
of halogenated, particularly chlorinated alkanes and/or alkenes have
received much attention. However a problem with these processes is that
during the process, water may be generated as a by-product from reaction
of hydrogen fluoride with the catalyst or as a product of catalyst
regeneration processes, or indeed, the hydrogen fluoride starting material
may contain small amounts of water. If steps are not taken to remove this
water, then the concentration of water will increase. Hydrogen
fluoride/water mixtures are especially corrosive, and are both difficult
and expensive to handle. Moreover water, even at low levels, may act as a
catalyst poison.
In the past it has been proposed to remove the water by providing a
distillation column which is dedicated to separating substantially
anhydrous hydrogen fluoride from a water/hydrogen fluoride mixture.
However, such a column must be made of exotic corrosion resistant
materials and is expensive.
We have now devised a process by which water may be removed from a process
stream and which is cheaper and simpler to operate and eliminates or at
least substantially reduces the need for a distillation column which is
dedicated to separating substantially anhydrous hydrogen fluoride from
water/hydrogen fluoride mixtures.
According to a first aspect of the invention there is provided a process
for the removal of water from a process stream which comprises hydrogen
fluoride, water, organic products and by-products and unreacted organic
starting materials which comprises (i) separating the process stream into
a lighter tops stream comprising hydrogen fluoride and lighter boiling
organic components from a heavy bottoms stream comprising hydrogen
fluoride, water and heavier organic components characterised in that (ii)
the heavy bottoms stream is fed to a phase separator under conditions of
temperature and pressure such that the heavy stream is in the liquid phase
and an organic fraction is separated from a hydrogen fluoride fraction
containing water and (iii) disposing of at least a part of the hydrogen
fluoride fraction.
We have realised that by performing step (i), which typically comprises
separation of a tops vapour from a bottoms liquid, usually by
distillation, a degree of separation of hydrogen fluoride from water is
achieved there by concentrating up the water in the hydrogen fluoride
bottoms phase such that when this bottoms phase undergoes phase
separation, the hydrogen fluoride phase contains significantly higher
concentration of water than the original process stream, there by allowing
the removal of less hydrogen fluoride with the water to be disposed of.
Typically the hydrogen fluoride/water fraction from the phase separator
will be divided into a recycle stream, which can be vaporised, to which
further hydrogen fluoride is added, and a smaller hydrogen fluoride/water
stream which may be further treated or disposed of, for example by being
sent to aqueous scrubbers.
The relative proportions of the fraction which is recycled and the fraction
which is sent to further treatment will depend particularly upon the rate
at which water is produced in the process, but usually the amount of the
hydrogen fluoride fraction disposed of will be such as to maintain the
concentration of water in the process stream prior to the present
invention at less than 0.5%, preferably less than 0.3% and especially less
than 0.2% by weight.
The processes from which the composition which is treated according to the
present invention is obtained are varied but they will typically be
catalytic vapour phase hydrofluorination reactions, and particularly the
vapour phase hydrofluornation of a halogenated alkane or alkene,
especially a chlorinated alkane or alkene. The process of the invention
may be advantageously employed with compositions obtained from the
reaction of hydrogen fluoride with a halogenated C1-C4 alkane or alkene in
the vapour phase and in the presence of a catalyst.
Particular processes from which a composition may be treated according to
the invention include production of HFA 134a, HFA 125, HFA 32, HFA 143a
etc. HFA 134a may be produced from 1,1,1 trifluoroethane 2-chloroethane
and/or trichloroethylene. HFA 125 may be produced from perchloroethylene.
HFA 32 may be produced from an .alpha.-fluroether, for example
bis-fluoromethlyether, or from methylene chloride. Conditions of
temperature and pressure, preferred fluorination catalysts, proportions of
reactants, the arrangement of reactors and methods of recovering pure HFA
product have been well documented and are well known in the art, for
example as described in EP 0 449 617 and EP 0 449 614 for HFA 134a, in
WO94/21579 and WO94/21580 for HFA 32, in WO92/16479 and WO94/16482 for HFA
125 and in EP 502 605 for vapour phase fluorinations generally, the
contents of all of which are hereby incorporated by reference.
For clarity, the invention will now be described with reference to a
composition which has been produced by the vapour phase fluorination of
perchloroethylene to produce pentafluoroethane, although the invention is
not so limited.
A further aspect of the invention provides a process for the production of
hydrofluorocarbon which comprises contacting a precursor compound with
hydrogen fluoride in the vapour phase in the presence of a
hydrofluorination catalyst to produce a product stream comprising the
hydrofluorocarbon, organic by-products, hydrogen fluoride and water and
treating at least part and preferably substantially all of the product
stream, optionally after prior treatment, in a process according to the
first aspect of the invention.
As desired, more than one hydrofluorocarbon may be produced in the process
by co-production with another hydrofluorocarbon. Suitably the precursor
compound for one or more hydrofluorocarbon products may be fed into the
phase separator or if present the recycle stream as desired for subsequent
fluorination to the hydrofluorocarbon product. By way of example, HFC 125
and HFC 134a may be co-produced by feeding perchioroethylene and
trichloroethylene into the phase separator and HFC 125 and HFC 32 may be
co-produced by feeding perchloroethylene and methylene chloride into the
separator.
The amount of water produced during these processes and thus the
concentration of water in the process off-gas depends in particular upon
the particular catalyst employed since certain catalysts have a tendency
to produce more by-product water than others. Thus, fluorination catalysts
which comprise a high proportion of metallic oxides will have a tendency
to produce more water than fluorination catalysts which contain a smaller
proportion of metallic oxides and more metallic halides. Although all
catalysts have a tendency to by-produce water during hydrofluorination
processes and more particularly during regeneration thereof, catalysts
which are based upon metal oxides, or mixed metal oxides, for example
chromia, alumina and other metallic oxides supported on chromia or
alumina, for example zinc, iron, magnesium, nickel tend to produce levels
of water which make it essential to provide a highly efficient water
removal process step.
In the hydrofluorination of perchloroethylene, we particularly prefer to
employ a catalyst as described in EP 0 502 605, the contents of which are
incorporated herein by reference.
The off-gas composition from the process typically comprises a major
proportion of hydrogen fluoride and hydrogen chloride, pentafluoroethane,
chlorotetrafluoroethane and dichlorotrifluoroethane together with minor
quantities of various chlorofluoroethane by-product impurities, unreacted
perchloroethylene and by-product water.
Prior to the process according to the invention, the stream needs to be
liquefied, and this may be achieved for example by distillation by partial
condensation, or by the use of a "quench" which is essentially a single or
multiple stage column to which no heat other than that of the reactor
off-gas fed to it, is input. During the liquefaction step, the volatile
components of the stream are removed from the top of a column as a vapour
and may then be sent to further purification stages to recover
pentafluoroethane. A cooled liquid is recovered from the bottom of the
column which comprises unreacted perchloroethylene, hydrogen fluoride,
water, dichlorotrifluoroethane, trichlorodifluoroethane and minor amounts
of unsaturated impurities. This liquid is then fed to the process of the
invention, and preferably a vessel in which the liquid is allowed to
reside for a time sufficient to allow satisfactory phase separation of the
liquid into a lower organic fraction which may be recycled to the process,
and an upper fraction containing mainly hydrogen fluoride in which the
water has effectively been significantly concentrated.
The conditions under which the process is effected are not critical
provided that the conditions are such that the stream to be phase
separated is in the liquid phase. It is convenient however to effect the
process of the invention at about atmospheric pressure or superatmospheric
pressures up to about 20 barg and preferably up to about 10 barg and at
ambient temperature, although temperatures in the range from -80.degree.
C. to 40.degree. C. or higher may be employed if desired, and
subatmospheric or superatmospheric pressures may also be employed.
The top hydrogen fluoride fraction containing water may then be divided
into a stream for further treatment and a stream which may be recycled to
the process. The proportion of the top fraction which is sent for further
treatment versus recycle depends upon the amount of water produced, the
efficiency of the phase separation and the concentration of water which
can be tolerated during the hydrofluorination process.
Generally, we prefer that the proportion which is recycled to the process
contains a concentration of water relative only to the hydrogen fluoride
present of less than 0.5%, preferable less than 0.3% and especially less
than 0.2%, and thus sufficient of the tops fraction is sent to further
treatment such that after the addition of further hydrogen fluoride to the
hydrogen fluoride/water which is recycled, the concentration of water
relative to hydrogen fluoride is within these limits. As a guide only,
this will usually require that between 2% and 5% of the tops fraction is
disposed of. This may be achieved simply by pumping the tops fraction out
of the vessel or allowing the tops fraction to drain from the vessel.
The invention is illustrated with reference to the following figures in
which:
FIG. 1 is a schematic flow-sheet of the process of the invention,
FIG. 2 is a schematic flow-sheet for the production of pentafluoroethane
from perchloethylene including the steps of distillation and phase
separation, and
FIG. 3 is a schematic flow-sheet for the production of pentafluoroethane
from perchloroethylene including the steps of distillation and phase
separation and in which an additional water/hydrogen fluoride distillation
step is also shown.
In FIG. 1, a typical reactor off-gas stream (1) from the vapour phase
hydrofluorination of perchloroethylene to produce pentafluoroethane over a
zinc on chromia catalyst and containing 23 kg/hr HF, 16 kg/hr HCl, 12
kg/hr HFA 125, 21 kg/hr HCFC 124, 15 kg/hr HCFC 123, 10 kg/hr
perchloroethylene and other minor components including HCFC 122, HCFC 1112
and 0.1 kg/hr of water is fed to a distillation column (2) in which a
lights stream (3) containing 16 kg/hr HCl, 12 kg/hr 125, 18 kg/hr124, 1
kg/hr 123 and 2 kg/hr HF is separated from a heavies stream (4) containing
14 kg/hr 123, 10 kg/hr Per, 21 kg/hr HF, and 0.1 kg/hr water. The latter
stream is fed via a cooler (10) to a phase separation vessel (5) in which
an organics rich phase (6) is separated from an HF rich phase containing
over 90% of the water (7). An HF/water purge stream (8) is taken from
stream (7) for further treatment, while the stream (9), containing at
least 90% by weight of stream (7) is recycled to the fluorination
reactors.
In FIG. 2, a fluorination reactor (14) is fed a stream (13) comprising Per
(11) and HF (12) feeds and recycle streams (24), (22) and (31). The
off-gas from the reactor, (15), is fed to a distillation column (16) in
which a lights stream (17) comprising mainly HCl, 125, 124 and HF, with
minor amounts of 114a and 133a, is separated from a heavies stream (18)
comprising HF, 123,122, Per, 1112, and other underfluorinated
intermediates to 125. This latter stream is fed to a cooler (19) before
entering a phase separation vessel (20) in which an HF rich stream (21) is
separated from an organics rich stream (24). The water content of stream
(18) is concentrated up in stream (21). Stream (22), containing the
majority of stream (21), is recycled to the reactors. Stream (23),
containing a small proportion of stream (21) is disposed of in aqueous
scrubbing system (25).
The lights stream (17) is fed to an aqueous scrubbing and drying stage
(25), in which HCl and HF is stripped from the organics. The acid free
organics are fed to compressor (26), prior to distillation in column (27)
in which a 125 stream (28) is separated from the 124 containing stream
(29). This stream is fed to a final distillation column, (30) in which a
124 recycle stream (31) is separated from stream (32) containing 114a and
133a. The latter stream is removed from the process for further treatment.
In FIG. 3, a fluorination reactor (36) is fed a stream (35) comprising Per
(33) and HF (34) feeds and recycle streams (44), (47) and (54). The
off-gas from the reactor, (37), is fed to a distillation column (38) in
which a lights stream (39) comprising mainly HCl, 125, 124 and HF, with
minor amounts of 114a and 133a, is separated from a heavies stream (40)
comprising HF, 123,122, Per, 1112, and other underfluorinated
intermediates to 125. This latter stream is fed to a cooler (41) before
entering a phase separation vessel (42) in which an HF rich stream (43) is
separated from an organics rich stream (44). Stream (43) is fed to 10
distillation column (45) in which an HF recycle stream (47) is separated
from a stream containing HF and water (46). The latter stream can be fed
to aqueous scrubbing system (48) in which the acid content is diluted
prior to neutralisation.
The lights stream (39) is fed to an aqueous scrubbing and H2SO4 drying
stage (48), in which HCl and HF is stripped from the organics. The acid
free organics are fed to compressor (49), prior to distillation in column
(50) in which a 125 stream (51) is separated from the 124 containing
stream (52). This stream is fed to a final distillation column, (53) in
which a 124 recycle stream (54) is separated from stream (55) containing
114a and 133a. The latter stream is removed from the process for further
treatment.
The advantage of this FIG. 3 scheme over the FIG. 2 scheme is that the HF
losses are minimised by further distillation of the HF/H20 stream to give
anhydrous HF and [as a limit] HF/H20 azeotrope; however the additional
capital cost of the HF still has to be offset against this improvement in
HF efficiency.
The invention is further illustrated but not limited by the following
examples.
EXAMPLE 1
Hydrogen fluoride, perchioroethylene and water were mixed in the quantity
by weight given in Table 1 below and were charged to a 300 ml FEP
(copolymer of tetrafluoroethylene and hexafluoropropylene) separating
vessel, mixed well and allowed to phase separate for about 10 minutes.
Samples of the lower perchloroethylene(per) rich phase and lower hydrogen
fluoride rich phase were taken and analysed for their water content by
Karl-Fisher titration.
The phase separation vessel was attached to a scrubbing train to allow
discharge of the denser per rich phase into a series of ice and water
scrubbers. The separation vessel was reweighed after discharge of the
perchloroethylene phase to give the weight of the perchloroethylene phase
(by difference) and the acid content of this perchloroethylene rich phase
was obtained by titration of the combined ice and water scrubber liquors.
Five runs were conducted following the above procedure but for each run a
different compositional mixture of perchloroethylene, hydrogen fluoride
and water (as detailed in Table 1 below) was employed, all runs being
conducted at 22.degree. C. and atmospheric pressure.
The experimental results are summarised in the Table 1 below.
As indicated in the Table below, H.sub.2 O is concentrated in the HF rich
phase, rather than the perchloroethylene rich phase.
TABLE 1
__________________________________________________________________________
Wt HF Wt H.sub.2 O
Wt of per
Wt of HF in per
H.sub.2 O in per
H.sub.2 O in HF
(g) Wt Per (g) (g) rich phase rich phase rich Phase (g) rich phase
__________________________________________________________________________
(g)
90 8.1 5 8 0.16 0.1 4.9
90 8.1 5 8 0.17 0.09 4.91
90 48.6 5 49 1 0.22 4.78
90 48.6 5 49 1.1 0.17 4.83
90 97.2 5 99 1.9 0.26 4.74
__________________________________________________________________________
EXAMPLE 2
Hydrogen fluoride perchloroethylene, HCFC 123 and water were mixed, in the
quantity by weight shown in Table 2 below, and were charged to a stainless
steel vessel. The vessel was shaken vigorously to ensure good mixing and
then allowed to stand for about 10 minutes while phase separation took
place.
Samples of the lower organic phase were taken and analysed for water and HF
content. The remainder of the organic phase was then discharged and the
vessel was then reweighed. This enabled the weights of both the organic
and inorganic phases to be calculated. Samples of the inorganic phase were
then taken and analysed for water content.
Two runs were conducted following this procedure, the ratio of
perchloroethylene to HCFC 123 being varied in each run. Both runs were
conducted at room temperature and around atmospheric pressure. The results
are summarised in Table 2 below.
These results show clearly that most of the water charged to the system is
concentrated into the HF-rich phase.
TABLE 2
__________________________________________________________________________
Wt Wt of
Wt HF HCFC H.sub.2 O Wt of HF in per H.sub.2 O in per H.sub.2 O in HF
(g) Wt Per (g) 123 (g) (g) rich phase
rich Phase (g) rich phase (g)
__________________________________________________________________________
48.5
36.4 12.1
0.4 0.08 0.08 0.32
48.5 12.2 36.6 0.36 0.04 0.08 0.28
__________________________________________________________________________
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